MnFe2O4-NH2-HKUST-1, MOF magnetic composite, as a novel sorbent for efficient dye removal: fabrication, characterization and isotherm studies

Dye in industrial wastewater is one of the most serious environmental concerns due to its potentially harmful effects on human health. Many industrial dyes are carcinogenic, toxic and teratogenic. Removal and recovery of hazardous dyes from the effluents requires efficient adsorbents. In this study, magnetic adsorbent MnFe2O4-NH2-HKUST-1 was synthesized to remove methylene blue and crystal violet dyes from aqueous solutions. The synthesized adsorbent was characterized using FTIR, XRD, BET, VSM, SEM, TGA and Zeta potential techniques. The effect of different parameters such as pH, contact time, and adsorbent dosage on the removal of dyes was investigated. The dye adsorption process was investigated by UV–Vis spectrophotometry. The maximum adsorbent capacity was obtained as 149.25 mg/g for methylene blue and 135.13 mg/g for crystal violet. The adsorption equilibrium isotherm and kinetic models were plotted and results showed that the adsorption process for both dyes is a collection of physical and chemical adsorption based on langmuir and freundlich isotherm models, and follows the pseudo-second-order adsorption kinetics. This study shows that magnetic adsorbent MnFe2O4-NH2-HKUST-1 has a good potential for removal of methylene blue and crystal violet dyes from water in a short time (5 min) and it is easily separated from the solution by a magnetic field due to its magnetic property.

The concentration of the dyes was determined using double-beam UV-Vis spectrophotometer (Lambda 35, Perkin-Elmer, USA).The samples were characterized by scanning electron microscopy (SEM) KYKY-EM 3200 with gold coating.pH of solutions was measured usin a glass pH electrode (metrohm 713 pH-meter).Powder X-ray diffraction (XRD) data were obtained on a Philips X'pert diffractometer with monochromatic Cu-Ka radiation.
FT-IR spectra were recorded on a Equinox 55 Bruker model FT-IR spectrophotometer using KBr pellets.Surface areas were determined by the BET (Belsorp mini II, Microtrace Bel Corop) method.The measurement of the magnetic field was done by Vibrating Sample Magnetometer (VAM LBKFB, Kavir Co).The thermogravimetric analysis (TGA) was carried out on a TGA Bahr, Germany Instrument and Zeta potential (SZ-100Z, Horiba Jobin Jyovin) analysis was done to determine the surface charge of the synthesized adsorbent.
Synthesis of MnFe 2 O 4 -NH 2 magnetic nanoparticle 0.8203 g (10 mmol) of sodium acetate was dissolved in 6.5 ml of ethylene glycol, then refluxed at 100 °C for 15 min.After that, 0.1709 g (0.66 mmol) of manganese(II) nitrate tetrahydrate and 0.5385 g (1.3 mmol) of iron(III) nitrate nonahydrate were dissolved in 3.5 ml of ethylene glycol and added to the previous solution and refluxed again for 30 min.Then 2.3 ml of ethanolamine was added to the resulting mixture and refluxed for 12 h at a temperature of 200 °C.After the completion of the reaction, the obtained product, which is a brown precipitate with magnetic properties, was cooled at room temperature and separated by a magnetic separator.Then it was washed several times with water and ethanol and finally it was dried at 70 °C for 4 h (Scheme 2).O in 25 ml of ethanol and placed in an ultrasonic bath for 30 min.After that, 0.21 g (1 mmol) of benzene 1, 3, 5 tricarboxylic acid ligand was dissolved in 25 ml of ethanol andadded to the previous solution under mechanical stirring at a rate of 0.5 ml/min for 1 h.The resulting green product was washed several times with ethanol, separated with a megnet and placed in vacuum oven at 50 °C for 4 h to dry (Scheme 3).

Removal experiments
At first, solutions of both MB and CV dyes were prepared in the concentration range of 1.28-10.00and 0.81-6.93mg/l in distilled water, respectively.Then, the adsorption process at the wavelength of 664 nm for MB and 590 nm for CV was investigated by adding 5 and 3 mg of adsorbent to 5 ml solutions of these dyes.After the adsorption process was completed, the synthesized adsorbent was removed from the solutions by an external magnet and the remaining concentrations of the dyes were calculated with the following equation: where A 0 is the initial adsorption of the solution and A is the adsorption of the analyte after adding the sorbent.
In the FT-IR spectra related to HKUST-1 bands in the regions of 1447 cm −1 and 1639 cm −1 are attributed to the carboxyl groups O-C-O and the bands that appeared in the regions of 1375 cm −1 and 1565 cm −1 are attributed to the stretching vibration of C=C in the BTC ligand.The band appearing in the region of 680 cm −1 is related to the Cu-O bond 18 .The bands appearing in the regions of 2925 cm −1 and 2854 cm −1 are attributed to the stretching vibration of the C-H bond in ethylene glycol or ethanolamine.The broad band in the region of 1049 cm −1 can be attributed to the overlap of the C-O bond with the stretching vibration of the C-N bond, which indicates the binding of amino groups on the MnFe 2 O 4 -NH 2 nanoparticle.The band in the range of 500-600 cm −1 is related to the Fe-O bond 19,20 .Also, the bands at 1385 cm −1 , 1627 cm −1 and 3421 cm −1 are respectively related to C-N stretching vibration, NH 2 scissor bending vibration and N-H stretching vibration, which is a sign of the presence of ethanolamine molecules on the nanoparticle surface 21 .The broad peak appearing in the region of 3418 cm −1 can be attributed to -OH water molecules in the structure of HKUST-1 11 .
The peaks in the examined patterns can be found in the diffractogram of the synthesis of the MnFe 2 O 4 -NH 2 -HKUST-1 adsorbent.

SEM micrographs
SEM analysis was used to determine the morphology and particle size of MnFe 2 O 4 -NH 2 nanoparticles and magnetic adsorbent MnFe 2 O 4 -NH 2 -HKUST-1.Fig. 2a shows metal oxide nanoparticles MnFe 2 O 4 -NH 2 in two magnifications.According to these images, metal oxide nanoparticles are spherical shape with an approximate diameter of 45 nm. Figure 2b is the recorded SEM images of MnFe 2 O 4 -NH 2 -HKUST-1 magnetic adsorbent octahedral particles, which seems that the nanoparticles are placed on the surface of HKUST-1.

EDX analysis
EDX analysis was used to semi-quantitatively identify the constituent elements of the sample.Figure 2c and d are the EDX analysis of MnFe 2 O 4 -NH 2 and MnFe 2 O 4 -NH 2 -HKUST-1, respectively.The presence of oxygen, iron and manganese elements in the spectrum of MnFe 2 O 4 -NH 2 and elements of carbon and copper in the spectrum of MnFe 2 O 4 -NH 2 -HKUST-1 in addition to the above elements are fully evident.

BET analysis
In order to determine the specific surface area of the magnetic adsorbent, the adsorption-desorption of nitrogen gas at the temperature of 77 K was used.The adsorption-desorption of N 2 is attributed to a type IV isotherm wich shows that the adsorbent has a mesoporous structure.According to the results obtained from this analysis, the synthesized MnFe 2 O 4 -NH 2 -HKUST-1 has a specific surface area equal to 333.46 m 2 g -1 and average pore diameter equal to 17.234 nm (Fig. 3a).

TGA analysis
To determine the thermal stability of MnFe 2 O 4 -NH 2 -HKUST-1 magnetic adsorbent, TGA analysis was performed.Figure 3b shows the changes in the mass.The first weight loss is observed (20%) in the temperature range of 80-150 °C, which can be attributed to the evaporation of water and other volatile substances in the adsorbent

VSM analysis
In order to determine the magnetic properties of MnFe 2 O 4 -NH 2 and MnFe 2 O 4 -NH 2 -HKUST-1, VSM analysis was performed.Figure 4a shows the hysteresis curves of these compound.According to the graph in Fig. 4a (I), the value of saturation magnetization of the MnFe 2 O 4 -NH 2 nanoparticle was 31.47 emu/g.Fig. 4a (II) is related to the saturation magnetization of MnFe 2 O 4 -NH 2 -HKUST-1 adsorbent, which is calculated as 7.70 emu/g and has decreased compared to the saturation magnetization of nanoparticles, which is due to the thickening of the non-magnetic component, however, the magnetic property of the synthesized adsorbent is favorable for its quick separation by magnet from the solution.According to the obtained results, the synthesized adsorbent has superparamagnetic properties and is well collected from the solution with an external magnet.

Zeta potential
Zeta potential analysis was used to determine the surface charge of the magnetic adsorbent.The result of the analysis is given in Fig. 4b, which shows that the surface charge of the adsorbent is negative and equal to − 39.8 mv, which according to this feature seems to have a good ability to remove positively charged species.www.nature.com/scientificreports/

Application of composite for removal of dyes
The efficiency of the synthesized adsorbent was checked by performing the adsorption process of two dyes, MB and CV, for this purpose, the adsorption spectrum was taken from the dyes before and after adding the adsorbent (Fig. 5a,b).
In order to increase the efficiency of the adsorption process, all parameters affecting the surface adsorption process such as solution pH, adsorbent weight and contact time were optimized.

Initial pH optimization
pH of the solution has an important effect on the interaction between the adsorbates and the adsorbents.In order to investigate the effect of this parameter, the dye adsorption process was carried out within the solution pH ranging from 3.0 to 10.0 and removal percentage was calculated based on Eq. (1) (Fig. 6a,b).The results showed that the maximum adsorption capacity of MnFe 2 O 4 -NH 2 -HKUST-1 adsorbent for MB and CV is in pH solution of 4.5 and 4, respectively.Variation in adsorption levels at different pH values can be attributed to the electrostatic interaction between the charged surface of the adsorbent and organic dyes 23 .In these pH values, the dyes are in their cationic forms and can adsorbed on the negative surface of sorbent (based on zeta potential analysis) due to the electrostatic interaction.A decrease in adsorption at high acidic media can be due to excessive protonation of the adsorbent surface, which gives it a positive charge.This leads to increased electrostatic repulsive interaction between the adsorbent surface and the positively charged MB and CV, resulting in reduced adsorption.At higher pH values, the concentration of OH -ions in the solution increseas.This leads to competition between the adsorbent surface and OH -ions for the MB and CV cations which results in a decrease in the adsorption.

Amount of sorbent optimization
A favorable adsorbent should have a suitable acceptance capacity for the analyte.In order to investigate the effect of adsorbent weight, the surface adsorption process for two dyes, MB and CV, was performed by adding 1-5 mg and 1-10 mg of adsorbent to 5 ml of 1.5 × 10 −5 and 1.3 × 10 −5 mol/L solution of MB and CV, respectively.The results showed that the optimal weight of the adsorbent for MB is 3 and for CV is 5 mg, which is shown in Fig. 6c and d.  www.nature.com/scientificreports/

Effect of adsorption time
Stirring time is one of the parameters affecting the adsorption process, which determines the kinetics test time.In order to determine the optimal time, the dye adsorption process was carried out at different times and resulting adsorption spectra of both dyes were examined.For both dyes, it was observed that after 5 minutes, the amount of adsorption was fixed.Therefore, this time was chosen as the optimal time for both dyes.

Reusability and reproducibility
Reusability studies showed that the synthesized adsorbent doesn't have much ability for repeated use and it was the major disadvantage of it.In order to check the reproducibility, four experiments were performed in optimal conditions for both MB and CV dyes.The results are reported in Table 1.

Adsorption capacity and isotherm
Surface adsorption isotherms are used to investigate the interaction between the adsorbates and the adsorbents..The linear form of the Langmuir model is given in Eq. ( 2): where q e (mg g −1 ) is the equilibrium adsorption capacity; C e (mg L −1 ) Final concentration of MB and CV; q max (mg g −1 ) is the maximum adsorption capacity; K L (L mg −1 or L mol −1 ) Langmuir or equilibrium constant for adsorption.K L and q max value can be calculated from the slope and intercept of the linear plot of 1/Q e versus 1/ C e as shown in Fig. 7.
The shape of the Langmuir model was calculated by using the separation factor R L , which is presented in the form of Eq. (3): Freundlich isotherm model.The freundlich model is presented to describe multilayer and physical adsorption and it is assumed that the surface of the adsorber is heterogeneous and at first the adsorption sites that give a stronger bond are filled 25 .The linear mode of this model is given in Eq. ( 4): where n and K F are the Freundlich constants and express the intensity and capacity of adsorption; 1/n is the heterogeneity.Freundlich equilibrium constants were obtained from the linear plot of log Q e versus log C e .
(2) Langmuir model : Temkin isotherm model.This model assumed that the heat of adsorption in different layers decreases linearly for all molecules due to the interactions of the adsorbate and adsorbent.The linear form of this model is given in Eq. ( 5): where B is constant related to the heat of adsorption and it is defined by the expression B = RT/b; b (J mol −1 ) is the temkin constant and indicates the adsorption temperature; T (K) absolute temperature; R (8.314 J/mol K) is the gas constant; A (L g −1 ) Temkin isotherm constant.The values B T and A T can be calculated from the slope and intercept of the plot of Q e versus log C e 26 .
Dubinin-Radushkevich (D-R) isotherm model.In this model, it is assumed that the characteristics of the adsorption curves are related to the porosity of the adsorbent and the adsorption process occurs on heterogeneous surfaces.By using this isotherm, the type of adsorption (physical-chemical) can be recognized 27 .The linear form of this model is in the form of Eq. ( 6): where q D (mg g -1 ) is the maximum adsorption capacity; β is the activity coefficient useful in obtaining the mean sorption energy E (kJ mol −1 ) and ε (J mol −1 ) is the Polanyi potential which can be correlated by the following equations: If the value of E is less than 40 (kJ mol −1 ), the process of adsorption is physical and values higher than that indicate chemical adsorption 28 .β and Q D were obtained from the slope and intercept of the plot of log Q e versus ε 2 .
As shown in Fig. 7 and Table 2, it can be seen that the value of the coefficient correlation (R 2 ) obtained for the Langmuir and freundlich is higher compared to other models.It could be deduced that these models are applicable in the studied concentration range.Based on the q max value obtained from the Langmuir isotherm, the maximum adsorption capacity of the adsorbent for MB and CV dyes were 149.25 mg g −1 and 135.13 mg g −1 , respectively.The values of R L obtained from this isotherm indicate the adsorption process is favorable for both dyes.The n values derived from the freundlich equation were 1.1885 for MB and 1.1399 for CV.The n value was greater than unity which indicates the favorable adsorption.The positive values obtained for the b parameter from the temkin isotherm indicates that the adsorption occurs during an exothermic process.The calculated mean sorption energy E from the D-R isotherm for MB and CV were higher than 40 kJ mol −1 , indicating chemical adsorption.
These results suggest that the adsorption of MB and CV on MnFe 2 O 4 -NH 2 -HKUST-1 is a collection of physical and chemical adsorption.www.nature.com/scientificreports/

Adsorption kinetics
Studying the kinetics of the adsorption process is necessary for a better understanding of the adsorption mechanis.In this study, two conventional kinetic models of pseudo-first-order and pseudo-second-order adsorption were investigated.Kinetic parameters were calculated from the following equations: where q e (mg g −1 ) and q t (mg g −1 ) are the amount at adsorption equilibrium and the amounts of MB and CV adsorbed at time t (min), respectively, and k 1 (min −1 ) and k 2 (g mg −1 min −1 ) are the pseudo-first-order and pseudo-second-order rate constants, respectively 29 .The values of the correlation coefficients of the models (Table 3) show that the adsorption of MB and CV on MnFe 2 O 4 -NH 2 -HKUST-1 follows the pseudo-second-order model.

Adsorption thermodynamics
In order to investigate the effect of temperature on the adsorption process of MB and CV by magnetic adsorbent and determine the thermodynamic parameters, the surface adsorption process was carried out under optimal conditions and at five different temperatures.The results showed that the removal percentage of these two dyes decreases with increasing temperature.The thermodynamic parameters of entropy change (ΔS°), free energy change (ΔG°) and enthalpy change (ΔH°) were computed with the following equations: where R (8.314 J mol −1 K) is the universal gas constant, K (L mol −1 ) is the adsorption equilibrium constant, T (K) temperature, q e the amount of MB or CV adsorbed and C e the MB or CV concentration in solution 30 .
The thermodynamic parameters were determined − 31975 and − 36210 (kJ mol −1 ) for ΔH and 88.395 and 107.35 (J mol −1 K −1 ) for ΔS for MB and CV, respectively.The calues of ΔH indicate that the adsorption process is exothermic which shows that the adsorption process is more favorable at lower temperatures.

Conclusion
In this research, MnFe 2 O 4 -NH 2 -HKUST-1 magnetic adsorbent was synthesized and used to remove MB and CV dyes from aqueous solution.FTIR, XRD, BET, VSM, SEM, TGA and Zeta potential analyzes were performed to determine the morphology and structure of the synthesized adsorbent.Surface adsorption experiments were conducted to remove these two dyes.The effect of different factors on the adsorption process such as pH, contact time and amount of adsorbent was investigated.pH 4.5 and pH 4 were chosen as the optimal pH to remove MB and CV, respectively.The results of the experiments showed that the recovery of dye removal for each color is high in very low time, 5 min.By examining the adsorption isotherms, it was concluded that the adsorption process of this compound on the adsorbent is a collection of physical and chemical adsorption.The maximum adsorption capacity for MB and CV was 149.25 mg g −1 and 135.13 mg g −1 , respectively.Thermodynamic investigations showed that the adsorption process of these dyes on the synthesized adsorbent is spontaneous and exothermic.The results of the experiments showed that the synthesized adsorbent has good magnetic properties that could be collected from the solution and has a good capacity to remove MB and CV from aqueous solutions in short time.(9)  Log q e − q t = Logq e − k 1 t 2.303
https://doi.org/10.1038/s41598-024-59727-8crystal.The second weight loss in the temperature range of 150-220 °C is due to the removal of solvent molecules and other unreacted organic chemicals in the adsorbent structure, and a significant weight loss at 350°C indicates the destruction of adsorbent organic ligands.As the temperature increases, only metal oxides such as CuO and Fe 2 O 3 remain and finally, the sample is completely decomposed.

Figure 5 .
Figure 5.The adsorption spectrum of (a) MB and (b) CV before and after adding the adsorbent.

Figure 6 .
Figure 6.The effect of pH on the adsorption process of (a) MB, (b) CV by the sorbent in the range of 3.0 to 10 and the graph of the adsorption percentage of (c) MB and (d) CV from the solution according to the weight of the sorbent.

Table 2 .
Isotherm constants for MB and CV.

Table 3 .
Pseudo-first-order and pseudo-second-order kinetic model.